The present invention relates generally to a braking device which allows active control of the brake-fluid pressure, and more particularly, to a brake control system having a brake line branched to wheel cylinders.
In recent years, vehicle-mounted braking devices have various functions added. Such braking devices are known, for example, in the form of an antiskid braking system for achieving shortening of the braking distance while maintaining the vehicle behavior stable by preventing wheel lock, a traction control system for preventing spin of driving wheels during vehicle acceleration and the like, a brake assist device constructed so that when the brake-fluid pressure operated by a driver is insufficient, this insufficiency of brake-fluid pressure is supplied to a wheel cylinder, and a vehicle stability control (VSC) system for securing the running stability of the vehicle by providing a braking force to the wheels regardless of presence of brake operation by a driver so as to remove oversteer or understeer of the vehicle.
The VSC system serves to automatically adjust, when the vehicle can oversteer or understeer, the brake-fluid pressure within the wheel cylinder without driver""s brake operation to produce in the vehicle a lateral force opposed to oversteer or understeer. Due to its purpose of stabilizing the vehicle behavior, the VSC system is required to react to the vehicle behavior in a very short time. That is, in contrast to the antilock braking system, the VSC system is required to carry out braking in a very short time.
Further, in the antilock braking system which assumes brake operation carried out by a driver, the target brake-fluid pressure within the wheel cylinder cannot exceed the brake-fluid pressure within the master cylinder. On the other hand, in the VSC system, since control is carried out regardless of the presence or absence of driver""s brake operation, the target brake-fluid pressure within the wheel cylinder is set to be higher than the brake-fluid pressure within the master cylinder, and is thus desired to be increased to a higher valve in a further short time.
Furthermore, when relatively great lateral acceleration is produced in the vehicle during cornering or the like and when the braking device is not actuated, this lateral acceleration affects a rotor of the braking device, which can be inclined or moved to the side of a brake piston, i.e. in the brake releasing direction. If the brake piston is pushed back in the brake releasing direction, and when the brake-fluid pressure is provided to the brake piston for brake actuation, the brake piston can make free run by an amount of this displacement, leading to delay of generation of a braking force. That is, as compared with the characteristic of brake-fluid pressure rise with no displacement of the brake piston, the responsivity of brake-fluid pressure rise can be degraded by an amount of the free-run section of the brake piston.
It is, therefore, an object of the present invention to provide a brake control system which contributes to an enhancement in the pressure-rise responsivity during active brake control.
The present invention provides generally a brake control system for a motor vehicle, which comprises: a source of a brake-fluid pressure; a brake line connected to the source; an OUT-side gate valve arranged in the brake line, the OUT-side gate valve serving to open and close the brake line; branch lines branched from the brake line downstream of the OUT-side gate valve to wheel cylinders; inlet valves arranged in the respective branch lines, each inlet valve serving to open and close the corresponding branch line; a drain circuit which serves to discharge the brake-fluid pressure within each wheel cylinder to a reservoir; outlet valves arranged in the drain circuit, each outlet valve serving to open and close the drain circuit; a pump which supplies the brake-fluid pressure to a supply line of the brake line downstream of the OUT-side gate valve; and an ECU which controls operation of the valves and the pump, the ECU being so constructed as to: supply the brake-fluid pressure to the supply line by driving the pump with an opening of the OUT-side gate valve restrained; carry out active brake control for independently controlling the brake-fluid pressures within the wheel cylinders by selectively opening and closing the inlet valves; return the brake-fluid pressures within the wheel cylinders to the source by opening the OUT-side gate valve at completion of active brake control; and carry out pressure-increase assist control for restraining the opening of the OUT-side gate valve and opening the inlet valves during a predetermined time when active brake control is switched from a state that it is effective for only one of the wheel cylinders to a state that it is effective for only another wheel cylinder.